1. Cross Reference to Related Commonly Owned Patent Applications and Rights
The present patent application is related to a patent application Ser. No. 865,231 entitled "Integrated Adaptive Optics Apparatus," by Thomas R. O'Meara and George C. Valley, filed on May 20, 1986, and to U.S. Pat. No. 4,019,807--Boswell et al., entitled "Reflective Liquid Crystal Light Valve with Hybrid Field Effect Mode", both assigned to Hughes Aircraft Company. The specifications of the related commonly owned patent application and patent are hereby incorporated by reference into the present application.
2. Field of the Invention
The present invention pertains to adaptive optics systems used in the correction and preconditioning of high energy laser beams. The specific focus of the preferred embodiment is the improvement in performance and spatial resolution of an adaptive optics correction system which utilizes a liquid crystal light valve.
3. Background Information
Since the invention of the laser in the 1950s, the optics industry has succeeded at vastly improving the power and utility of coherent light sources. The amount of radiant energy which can be transmitted over great distances with a minimum of scattering and diffraction losses has increased dramatically. In addition, a great number of applications have been developed for exploiting the spectral purity and spatial coherence of the laser beam. Communications, data transfer, and the projection and processing of images have come to depend upon the unique properties of the coherent laser wavefront. These properties must be preserved if the powerful and beneficial qualities of laser radiation are to be fully utilized. Except in free space, a laser beam travels through a material medium. When a laser beam propagates through glass, salt, or quartz lens arrangements; optical fibers; or the atmosphere, the wavefront quality of the laser beam is reduced. High spatial quality waves, upon traversing such optical systems, become aberrated; a plane wave emerges with a randomly perturbed wavefront. The diffraction associated with such aberrated waves significantly reduces the ability to focus the beam to a high-quality beamspot or to efficiently transmit a communications signal to a remote receiver.
Another problem occurs when such laser wavefronts are transmitting large amounts of energy. Some portion of that energy is absorbed when it passes through a given optics system of lenses, mirrors, and other optical devices, or travels through the atmosphere. Typically, when materials absorb energy and heat up, their index of refraction changes. This change in index varies across a given beam profile. The intensity of the beam and the amount of heat absorbed vary as a function of location within the beam. Differences in index cause refraction of a laser beam, just as the heated air above the desert bends the image of the sky into a mirage. The consequent spreading of the high-energy laser beam due to a laser-induced index differential is termed "thermal blooming". For a laser system intended to deliver an appreciable amount of energy over a long distance, such an effect is disastrous. The beam which arrives at its target has spread too far, and even if it were to be focused, the phase of the beam across its wavefront is so randomized that on the whole it destructively interferes and cancels itself out. The laser beam delivers only a small fraction of the excited laser medium.
In order to counteract these deleterious effects of atmospheric turbulence, thermal blooming, and irregularities within the optical train, adaptive optical systems have been explored and developed. These systems combine wavefront sensing and wavefront correction within a closed feedback loop in order to correct a particular laser beam's wavefront errors. A typical laser beam direction system might work as follows. A laser beam is directed via an atmospheric path to a target or receiving site. Because of turbulence and thermal blooming, only a portion of the radiation reaches the target. With cooperative systems, a laser reference is transmitted back through the atmosphere in order to be used as a probe wave which samples the atmospheric aberrations the light has encountered. Corner reflectors or target glints in uncooperative systems can reflect impinging laser radiation to achieve the same result. In essence, the return signal contains in its wavefront phase all the aberrations of the beam path. If the phase aberrations are then sensed and the laser beam is pre-aberrated to correspond to this phase pattern, during its propagation through the atmosphere the laser will retrace the path of the target radiation and arrive at the target unaberrated: the full amount of beam energy will then been transferred.
A variety of apparatus and methods have been advanced for this type of beam correction. The deformable mirror is perhaps the most popular and most easily understood. The deformable mirror is composed of a thin flexible glass, metallic sheet, or metallized membrane behind which is an array of piezoelectric or solenoid actuators. These actuators are push-pull devices which deform the mirror surface from its normal planar state. The reference radiation returning to the laser aiming system strikes this deformable mirror, passes through the adaptive optics system, and the wavefront's phase aberrations are measured by any one of a number of standard techniques which are well known to those persons skilled in the art. This phase information, transformed to electronic signals by a wavefront error sensor and operated upon by electronic signal processors and computer algorithms, governs the voltages to be applied to each actuator. The system continuously adjusts the mirror's front contour until the return target radiation is restored to a perfect wavefront. Then, the laser radiation reflected by this deformed mirror is the time-reversed phase conjugate of the return target radiation and arrives at the target almost completely unaberrated, despite thousands of kilometers of atmospheric turbulence and thermal effects.
The deformable mirror possesses a great number of inherent problems. The use of discrete, bulky electrical actuators limits the spatial frequency response for the mirror; a deformable mirror simply can not correct errors finer than the spacing of the push-pull actuator elements. In addition, such actuators typically require several thousand volts for operation and are subject to arcovers and permanent breakdowns. Their impedance combined with the mass of the mirror surface limits the temporal frequency response of the adaptive system. Each detectoractuator feedback loop requires discrete electronic processing systems and considerable amplification to function. Since the thin front surface of the mirror continuously experiences flexures, it suffers from eventual drift and creep problems with consequent loss in performance.
In an attempt to improve upon the deformable mirror's performance, other phase-conjugation approaches have been attempted. Nonlinear optical media, using stimulated Brillouin or Raman scattering, can provide a time-reversed wave as an output in some applications. In these methods, the medium is pumped by one or more local reference lasers and the electric field of the return distorted target radiation, upon entering the phase-conjugation cell, causes index variations within the nonlinear medium in exact correspondence to the interference pattern between the remote reference beam and the local pump beams. A high-energy laser reflecting off this index grating has its phase altered exactly as a phase-conjugate of the target beam. Again the laser propagates through the atmosphere with aberrations already corrected.
While this phase-conjugation method greatly improves the spatial resolution of the system (the "actuator spacing" is now molecular) it also possesses serious problems. The input sensitivity is very low, so that a fairly large return signal is necessary in order to set up the proper index grating structure within the phase-conjugation cell. Such power requirements rule out this method for lower-power optical communication and data transfer systems. In addition, the pump beams require an enormous amount of energy and must be precisely aligned for the device to function. The wasted costs of duplicate high-energy lasers for pumping the medium and the resulting low efficiency conversion and transmission of the energy to the target make phase conjugation an interesting but often impracticable means for adaptive optics applications.
None of the methods or devices described above provides an efficient and comprehensive solution to the problem of correcting the phase of an aberrated light wave. None of these methods provides a versatile, highly sensitive, compact, and simple apparatus for adaptively correcting coherent light wavefronts. An effective solution to this problem would satisfy a long felt need experienced by the optical community for over two decades. A truly practical and reliable means for precisely correcting the phase of a given light wave would represent a major advancement in the field of adaptive optics. Utilization of such a device within laser beam direction systems would enable extraordinarily complete wavefront improvement of system and atmospheric distortions. Such an invention would ideally be suited to operate in cooperation with a wide variety of adaptive optics systems and enhance any optical apparatus requiring high-resolution wavefront correction.